Sluggish form of electricity (6)
I believe the answer is:
static
This clue might be a double definition.
'sluggish' is the first definition.
(thesaurus)
'form of electricity' is the second definition.
'static' can be an answer for 'electricity' (static electricity is a kind of electricity). I am not sure about the 'form of' bit.
Can you help me to learn more?
(Other definitions for static that I've seen before include "irritation for the listener?" , "Constant interference" , "Still - type of electricity" , "Still, not moving" , "Atmospheric disturbance" .)